1. Field of the Invention
The present invention relates to an electronic telephone terminal (hereinafter referred to as the terminal) and, more particularly, it relates to such a terminal having an improved noise suppression function in a transmission system thereof.
In general, such a terminal is constituted by a reception system having an amplifier and a telephone receiver, and a transmission system having a transmitter (usually a microphone) and an amplifier.
In the terminal, particularly, in the transmission system according to the present invention, incoming noise received by the microphone from the surrounding environment can be effectively suppressed even when numerous.
2. Description of the Related Art
Conventionally, a carbon type microphone is widely used as a telephone transmitter, since the carbon microphone has many advantages. For example, the carbon microphone features a simple and solid structure and an effective noise suppression effect. The noise suppression effect is particularly important in this type of transmitter. For example, when the input sound pressure level is 90 to 100 dB, the sensitivity (gain) of the carbon type microphone to the input sound becomes 50 dB (where, 0 dB is .sqroot.watt/.mu.bar or .sqroot.W/0.1 Pa). But when the surrounding noise below is under 70 dB, the gain of the carbon type to the surrounding noise becomes about 40 dB. Accordingly, the apparent noise-to-signal ratio (S/N) is improved to the extent of 5 to 10 dB, since the noise from the surrounding environment is suppressed.
Although the noise suppression effect in a carbon type microphone can be obtained by non-linearity at the acoustic-to-electric transduction, this type of microphone has many drawbacks. For example the drawback include, distortion of the signal and unstable sound level, both caused by the above non-linearity. Further, it is difficult to control the quality of the carbon powders included in the microphone.
Recently, various electronic transmitting techniques which utilize, for example, a condenser type or a piezo-electric type microphone and an amplifier consisting of an integrated circuit (IC), have been widely employed in the terminal. In particular, these types of microphones are advantageous from the viewpoint of miniaturization and cost reductions. Further, by using these microphones, the non-linearity of the transmission system can be eliminated, and the distortion of the signal also can be reduced, as these microphones have a full linear characteristic.
Nevertheless, these microphones do not have a noise suppression function. Accordingly, the transmission sound is considerably affected by the surrounding noise, due to the full linear characteristic of the microphones, as mentioned above. Therefore, an IC amplifier having the non-linearity characteristic has been developed to solve the above problems.
Various terminals having a noise suppression function have been disclosed in, for example, the Journal of Technical Disclosure (No. 83-4877, published on May 20, 1983, by JIII), Japanese Unexamined Patent Publication (Kokai) No. 60-126950, and Japanese Examined Patent Publication (Kokai) No. 61-99451.
The first reference (No. 83-4877) discloses an automatic volume adjusting circuit for automatically adjusting the sound volume of a radio, a stereo, a television set, etc., in response to a surrounding noise level detected by a noise detector. But this reference merely discloses a volume adjusting technique and does not control the transmission sound level in response to the surrounding noise level.
The second reference (No. 60-126950) discloses a loudspeaker telephone terminal for detecting the surrounding noise level at a microphone, attenuating the noise level by a variable attenuator, and easily switching between the transmission system and the reception system. This reference, however, merely discloses a technique for easy switching of the sound switch means in the case of a large amount of surrounding noise.
The third reference (No. 61-99451) discloses a telephone terminal using a ceramic type microphone, instead of the carbon type. In this reference, when the level of the input sound is below a threshold value, the output level is lowered to make the characteristic thereof equal to that of the carbon type. In this reference, however, the variable attenuator is not controlled in response to the detected surrounding noise level. Furthermore, this control does not take into account relationship between the input sound pressure level and the surrounding noise level.